1. Field of the Invention
The present invention relates to solid state imaging systems and more particularly to the suppression of pattern noise in solid state imaging systems.
2. Description of the Prior Art
Solid state imagers, including both charge injection devices and charge coupled devices, consist of arrays of sensors in which rows and columns are activated in a regular manner by two separate series of pulses which scan the array. The horizontal scanning is done by a pulse which is stepped by a horizontal shift register along each sensor element in the row. This pulse creates a transient, which is the principal source of pattern noise. During the time period allocated for reading out the video signal stored on an element, the transient decays. When a long period can be allocated to the readout of each element, the video signal can be sampled late enough to allow a substantial decay in the transient and the derivation of a relatively noise free signal. As arrays have achieved higher resolution, however, the time allocated for reading each element has diminished, and other techniques to reduce the pattern noise have become necessary.
It has been recognized that the pattern noise is duplicated as row after row of sensors is read out, the pattern noise being primarily a function of parasitic capacities in the individual stages of the horizontal shift registers and in the enabling gates associated with each column of sensors. In other words, the pattern noise transients formed in all sensor elements tied to the same stage of the horizontal shift register and to the same horizontal enabling gates are substantially identical. At the same time, there is a substantial variation in the pattern noise transients from element to element in the same horizontal row of sensors.
Several techniques, in addition to delayed sampling, have been proposed to reduce the pattern noise. Since the pulses have an average dc content, it is known to add a fixed sample of the horizontal switching pulse in opposite polarity and suitable magnitude to effect a first reduction in the average amount of pattern noise. It is also known to filter out high frequency components of the transient. These techniques lead to a signal to noise ratio on the order of 10 or 20 to 1. Better signal to noise ratios are desirable.